From 81bf0016b3a776732bba430872ce5719f3251e70 Mon Sep 17 00:00:00 2001
From: Brian Picciano <me@mediocregopher.com>
Date: Mon, 4 Nov 2024 20:00:57 +0100
Subject: Add heading_template to gemtext middleware

---
 README.md                   | 11 +++++++++++
 http/handlers/gemtext.go    | 34 +++++++++++++++++++++++++++++++++-
 internal/gemtext/gemtext.go | 38 +++++++++++++++++++++++++++-----------
 3 files changed, 71 insertions(+), 12 deletions(-)

diff --git a/README.md b/README.md
index 6293bd8..829157a 100644
--- a/README.md
+++ b/README.md
@@ -65,6 +65,17 @@ The template will be rendered with these extra data fields:
 
 * `.Body`: A string containing all rendered HTML DOM elements.
 
+**heading_template**
+
+Path to a template which will be used for rendering headings. If not given then
+headings will be rendered with appropriate HTML header tags.
+
+The template will be rendered with these extra data fields:
+
+* `.Level`: Which level of heading is being rendered, 1, 2, or 3.
+
+* `.Text`: The text of the heading.
+
 **link_template**
 
 Path to a template which will be used for rendering links. If not given then
diff --git a/http/handlers/gemtext.go b/http/handlers/gemtext.go
index bc617f4..7f12799 100644
--- a/http/handlers/gemtext.go
+++ b/http/handlers/gemtext.go
@@ -64,6 +64,20 @@ type Gemtext struct {
 	//
 	TemplatePath string `json:"template"`
 
+	// Path to a template which will be used for rendering headings. If not
+	// given then headings will be rendered with appropriate HTML header tags.
+	//
+	// The template will be rendered with these extra data fields:
+	//
+	// ##### `.Level`
+	//
+	// Which level of heading is being rendered, 1, 2, or 3.
+	//
+	// ##### `.Text`
+	//
+	// The text of the heading.
+	HeadingTemplatePath string `json:"heading_template"`
+
 	// Path to a template which will be used for rendering links. If not given
 	// then links will be rendered using an anchor tag wrapped in a paragraph
 	// tag.
@@ -181,9 +195,23 @@ func (g *Gemtext) ServeHTTP(
 			Req:        r,
 			RespHeader: templates.WrappedHeader{Header: rec.Header()},
 		}
+
+		parser gemtext.HTMLTranslator
 	)
 
-	parser := gemtext.HTMLTranslator{}
+	if g.HeadingTemplatePath != "" {
+		parser.RenderHeading = func(w io.Writer, level int, text string) error {
+			payload := struct {
+				*templates.TemplateContext
+				Level int
+				Text  string
+			}{
+				ctx, level, text,
+			}
+
+			return g.render(w, ctx, osFS, g.HeadingTemplatePath, payload)
+		}
+	}
 
 	if g.LinkTemplatePath != "" {
 		parser.RenderLink = func(w io.Writer, url, label string) error {
@@ -258,6 +286,10 @@ func gemtextParseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler,
 			if !h.Args(&g.TemplatePath) {
 				return nil, h.ArgErr()
 			}
+		case "heading_template":
+			if !h.Args(&g.HeadingTemplatePath) {
+				return nil, h.ArgErr()
+			}
 		case "link_template":
 			if !h.Args(&g.LinkTemplatePath) {
 				return nil, h.ArgErr()
diff --git a/internal/gemtext/gemtext.go b/internal/gemtext/gemtext.go
index 76a990e..2b8cf77 100644
--- a/internal/gemtext/gemtext.go
+++ b/internal/gemtext/gemtext.go
@@ -14,6 +14,11 @@ import (
 // HTMLTranslator is used to translate a gemtext file into equivalent HTML DOM
 // elements.
 type HTMLTranslator struct {
+	// RenderHeading, if given can be used to override how headings are
+	// rendered. The level indicates which heading level is being rendered: 1,
+	// 2, or 3.
+	RenderHeading func(w io.Writer, level int, text string) error
+
 	// RenderLink, if given, can be used to override how links are rendered.
 	RenderLink func(w io.Writer, url, label string) error
 }
@@ -45,7 +50,7 @@ func (t HTMLTranslator) Translate(src io.Reader) (HTML, error) {
 		if writeErr != nil {
 			return
 		}
-		fmt.Fprintf(w, fmtStr, args...)
+		_, writeErr = fmt.Fprintf(w, fmtStr, args...)
 	}
 
 loop:
@@ -109,25 +114,36 @@ loop:
 			if t.RenderLink == nil {
 				write("<p><a href=\"%s\">%s</a></p>\n", urlStr, label)
 			} else {
-				if err := t.RenderLink(w, urlStr, label); err != nil {
-					return HTML{}, fmt.Errorf(
-						"rendering link %q (label:%q): %w", urlStr, label, err,
-					)
-				}
+				writeErr = t.RenderLink(w, urlStr, label)
 			}
 
 		case strings.HasPrefix(line, "###"):
-			write("<h3>%s</h3>\n", sanitizeText(line[3:]))
+			text := sanitizeText(line[3:])
+			if t.RenderHeading == nil {
+				write("<h3>%s</h3>\n", text)
+			} else {
+				writeErr = t.RenderHeading(w, 3, text)
+			}
 
 		case strings.HasPrefix(line, "##"):
-			write("<h2>%s</h2>\n", sanitizeText(line[2:]))
+			text := sanitizeText(line[2:])
+			if t.RenderHeading == nil {
+				write("<h2>%s</h2>\n", text)
+			} else {
+				writeErr = t.RenderHeading(w, 2, text)
+			}
 
 		case strings.HasPrefix(line, "#"):
-			line = sanitizeText(line[1:])
+			text := sanitizeText(line[1:])
 			if title == "" {
-				title = line
+				title = text
+			}
+
+			if t.RenderHeading == nil {
+				write("<h1>%s</h1>\n", text)
+			} else {
+				writeErr = t.RenderHeading(w, 1, text)
 			}
-			write("<h1>%s</h1>\n", line)
 
 		case strings.HasPrefix(line, ">"):
 			write("<blockquote>%s</blockquote>\n", sanitizeText(line[1:]))
-- 
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